The present invention relates to a process and a plant for the continuous hot dip-coating of a metal strip, especially a steel strip.
In many industrial applications, steel sheet is used which is coated with a protective layer, for example for corrosion protection, and usually coated with a zinc layer.
This type of sheet is used in various industries to produce all kinds of parts, in particular visual parts.
To obtain this kind of sheet, continuous dip-coating plants are used in which a steel strip is immersed in a bath of molten metal, for example zinc, which may contain other chemical elements, such as aluminium and iron, and possible addition elements such as, for example, lead, antimony, etc. The temperature of the bath depends on the nature of the metal, and in the case of zinc the temperature of the bath is around 460° C.
In the particular case of hot galvanising, as the steel strip runs through the molten zinc bath, an Fe—Zn—Al intermetallic alloy with a thickness of a few tens of nanometres forms on the surface of the said strip.
The corrosion resistance of the parts thus coated is provided by the zinc, the thickness of which is controlled usually by air wiping. The adhesion of the zinc to the metal strip is provided by the layer of the aforementioned intermetallic alloy.
Before the steel strip passes through the molten metal bath, this steel strip firstly runs through an annealing furnace in a reducing atmosphere where the purpose is to recrystallise it after the substantial work hardening resulting from the cold-rolling operation and to prepare its surface chemical state so as to favour the chemical reactions necessary for the actual dip-coating operation. The steel strip is heated to about 650 to 900° C. depending on the grade, for the time needed for recrytallisation and surface preparation. It is then cooled to a temperature close to that of the bath of molten metal by means of heat exchangers.
After it has passed through the annealing furnace, the steel strip runs through a duct, also called a “snout”, containing an atmosphere which protects the steel, and is immersed in the bath of molten metal.
The lower part of the duct is immersed in the bath of metal in order to define, with the surface of the said bath and inside this duct, a liquid seal through which the steel sheet passes as it runs through the said duct.
The steel strip is deflected by a roller immersed in the zinc bath. It emerges from this metal bath and then passes through wiping means used to regulate the thickness of the liquid metal coating on this steel strip.
At the moment when the strip is extracted from the bath, it passes through the surface of the zinc bath, which is covered with zinc oxide and with dross coming from the steel strip dissolution reaction.
To prevent the particles from being entrained by the strip, the surface of the bath, accessible by the operators, is periodically cleaned in such a way that the strip does not entrain particles.
However, this manual cleaning procedure does not permanently guarantee the cleanliness of the surface of the bath and the absence of particles periodically rising from the bath to the point where the steel strip is extracted.
Thus, the coated steel strip has visual defects which are magnified or revealed during the zinc wiping operation.
This is because the foreign particles are retained by the air wiping jets before the said particles are ejected or broken up, thus creating streaks of lesser thickness in the liquid zinc having a length ranging from a few millimetres to a few centimetres.
One solution for avoiding these drawbacks consists in cleaning the surface of the liquid seal by pumping off the zinc oxides and dross coming from the bath.
These pumping operations allow the surface of the liquid seal to be cleaned only very locally at the point of pumping and their effectiveness and range of action are very low, which does not guarantee that in particular the region where the steel strip leaves the liquid zinc bath is completely cleaned.